In the printing industry, it is well known to imprint a moving web of paper using an ink comprising a pigment and a volatile solvent. It will, of course, be recognized that although these printing inks will usually be based upon an organic solvent, the term "volatile solvent" also includes a water-based solvent under proper conditions of temperature and pressure. After such imprinting of the web, it is necessary to drive off the volatile solvent so that the ink will affix itself to the web. The web operates at a rapid speed, that is, in the range of from 300 to upwards of 2500 feet per minute, at a web tension in the range of 1 to 10 pounds per inch of web width, and the web speed and tension must be controlled by control rolls, so it is essential that the drying of the web and, consequently, the driving off of the volatile solvent, be achieved as quickly as possible. In the known art, it is common practice to run the printed web through a dryer in which jets of hot air impinge upon the respective planar surfaces of the web to heat the web and evaporate the volatile solvent. Shortly beyond the exit of the dryer, it is desirable to pass the web over one or more chill rolls to reduce the temperature of the web and to "fix" the inks imprinted upon the web. Condensation of the solvent on the chill rolls would cause smearing of the inks on the web. It is necessary, therefore, to remove as much of the solvent-laden boundary layer adhering to each planar surface of the web as is economically possible, so that a minimal amount of solvent is available to condense upon the chill rolls.
It is well known that when a flat sheet (such as a web of paper) moves relative to a fluid (such as the air through which the web moves), a stagnant boundary layer of the fluid adheres to the area near the surface of the flat sheet and that boundary layer is pulled along with the sheet or web as it moves relative to the bulk fluid. Although such boundary layers are usually considered with respect to flow of a fluid past a fixed flat sheet, for example, the flow of fluid past a stationary heat transfer surface, the situation for movement of a flat sheet, such as the web, through a fluid is analogous. It is the relative velocity difference between the fluid and the flat sheet that is relevant in determining the thickness of the boundary layer. Other important factors, all of which are clearly known and explained in the prior art, include the fluid viscosity and density. Both of these are, in turn, related to the chemical composition of the fluid, as well as its temperature. The specific calculation of the thickness of the boundary layer is well known in the art and a discussion of such calculation is presented in treatises such as Perry's "Chemical Engineer's Handbook", 5th ed., McGraw-Hill, 1973, at pages 5-55 through 5-57.
It is known in the prior art, particularly in the present inventor's U.S. Pat. No. 4,476,636, issued Oct. 16, 1984, which is incorporated herein as if fully recited, to modify the boundary air layer for a heat transfer roll. A further application of boundary layer effects is presented in U.S. Pat. No. 4,774,771, issued to Littleton on Oct. 4, 1988.
Since a web, particularly a paper web in the printing application described above, exits a hot air dryer at the high velocities commercially encountered, the web will, absent any control mechanism, carry a significant amount of a hot solvent-laden vapor with it as it exits the dryer. This has several undesirable aspects. For example, some of the common solvents, due to their chemical composition, are known to be deleterious to human health and the exposure of workers to the vapors may be limited by environmental standards. Secondly, the loss of heated air out of the exit of the dryer is also undesirable, particularly when such loss is in the stagnant boundary layer near the web, as the hot boundary layer reduces conductive heat loss from the web to the ambient environment. Even further, if the solvent-laden boundary layer is not wiped or otherwise removed prior to the encounter of the web with the chill rolls, the relative saturation of the solvent in the boundary layer may result in condensation of the solvent upon the chill rolls. In addition to decreasing contact between the web and chill roll, these solvents can smear or smudge the imprinting on the web. Although the thickness of the boundary layer so removed from the dryer is very small in relation to the width and length of the web, it will be easily recognized that the total volume of fluid thus removed is rather large per unit time, due to the high velocities utilized.